Impact of Oxygen Partial Pressure on the Ruddlesden−Popper Series Nd2-2xSr1+2xMn2O7: Oxygen Vacancy Formation and Ordering

The phase diagram of the Nd 2 - 2 x Sr 1+2 x Mn 2 O 7+ δ n = 2 Ruddlesden−Popper series has been investigated as a function of composition and oxygen partial pressure at 1400 °C. The regions of phase stability for 0.10 ≤ x ≤ 0.70 and 10 - 6 ≤ p (O 2 )/atm ≤ 1.0 have been mapped out and a systematic relationship between x and oxygen partial pressure established. At high p (O 2 ) (near 1 atm) the upper solubility limit is fixed by an average Mn oxidation state of +3.5, while the lower limit of stability is a strong function of p (O 2 ). Phases synthesized at low p (O 2 ) are associated with the introduction of oxygen vacancies that can be filled at low temperature to yield metastable phases with formal Mn oxidation states beyond +3.5. The oxygen vacancy structures are highly dependent on the Nd/Sr ratio, with the location of the vacancies changing from in-plane to axial as a function of both x and temperature. Systematic study of the Mn−O bonding framework reveals a subtle dependence of the crystal structure on both the Mn oxidation state and vacancy location. The unusual crystal chemistry of these phases suggests a more general synthetic technique for accessing layered manganite phases as potential CMR candidates.